1. Field of the Invention
The present invention relates to radio frequency identification (RFID) tags, and more particularly, to RFID tags that employ monopole antennas.
2. Description of Related Art
Radio Frequency Identification (RFID) transponders (tags) are operated in conjunction with RFID base stations for a variety of inventory-control, security and other purposes. Typically an item having a tag associated with it, for example, a container with a tag placed inside it, is brought into a xe2x80x9cread zonexe2x80x9d established by the base station. The RFID base station transmits an interrogating RF signal that is modulated by a receiving tag. That is, the RFID base station generates a continuous wave electromagnetic disturbance at a carrier frequency and this disturbance is modulated to correspond to data that is to be communicated via the disturbance. The modulated disturbance, or signal, communicates the information at a rate, referred to as the data rate, which is lower than the carrier frequency. The receiving tag modulates the RF signal in order to impart to the signal information stored within the tag and then transmits the modulated, answering, RF signal to the base station.
RFID tags may be active, containing their own RF transmitter, or passive, having no transmitter. Passive tags, i.e., tags that rely upon modulated back-scattering to provide a return link to an interrogating base station, may include their own power sources, such as a batteries, or they may be xe2x80x9cfield-poweredxe2x80x9d, whereby they obtain their operating power by rectifying an interrogating RF signal. Although both battery-powered and field-powered tags have minimum RF field strength read requirements, or read thresholds, in general, the range of a field-powered passive system or tag is limited by the amount of radiated power supplied from the reader and the range of a tag having its own power source is limited by the signal to noise level of the total communications link. Because the interrogating signal must provide power to a field-powered passive tag, the read threshold for a field-powered passive tag is typically substantially higher than for an active tag. However, because field-powered passive tags do not include their own power source, they may be substantially less expensive than active tags and because they have no battery to xe2x80x9crun downxe2x80x9d, field-powered passive tags may be more reliable in the long term than active tags. And, because they do not include a battery, field-powered passive tags are typically much more xe2x80x9cenvironmentally-friendlyxe2x80x9d.
Although field-powered passive tag RFID systems provide cost, reliability, and environmental benefits, there are obstacles to the efficient operation of field-powered passive tag RFID systems. In particular, it is often difficult to deliver sufficient power from a base station to a field-powered passive tag via an interrogating signal. The amount of power a base station may impart to a signal is limited by a number of factors, not the least of which is regulation by the Federal Communication Commission (FCC). An RFID tag may employ a resonant antenna in order to best utilize the signal power available to it. Unfortunately, a resonant antenna may require a good deal more area than is available to an RFID tag in many applications.
Consequently, it would be highly advantageous to provide an RFID tag that is compact, light weight, low cost and that efficiently employs a substantial portion of the signal energy which it receives from an interrogating base station.
A radio-frequency identification (RFID) transponder (tag) in accordance with the principles of the invention includes a monopole antenna that is xe2x80x9chollowed outxe2x80x9d to extend the electrical length of the antenna. In this manner, an antenna having a given electrical length may be formed within a much more compact area than a conventional antenna.
In an illustrative embodiment of the invention, an RFID tag employs an antenna formed of conductive material deposited or plated on a substrate and having generally triangular-shaped interior and exterior perimeters. The electrical length of the thus-formed antenna is significantly greater than that of a solid triangular shaped antenna having the same overall xe2x80x9cfootprintxe2x80x9d. Because the removal of interior conductive material tends to reduce the efficiency of the antenna (when compared with its continuous conductive sheet counterpart), the amount of interior conductive material removed may be limited to that amount which yields a predetermined antenna efficiency. Further increases in the electrical length and, consequently, reduction in overall antenna area, may be achieved by curving one or both of the antenna perimeters into generally scalloped or serpentine shapes.
In another aspect of the invention, all components of the tag may be situated on one side of a substrate, with a quarter wave stub acting as a reference. In an illustrative embodiment, the RFID tag includes a substrate material, such as a rigid material such as glass, ceramic, or printed circuit board material, or a thin, flexible material, such as organic materials such as polyimide or paper, or plastic, having an integrated circuit (tag IC) mounted on one surface of the substrate. The tag IC includes RFID tag electronics and may be thinned by grinding, etching, or polishing away a portion of the tag IC""s substrate material. The tag IC is connected at one terminal to the new hollowed monopole antenna and at another terminal to a quarter wave transformer, which operates as a reference. The resulting RFID tag is simple, inexpensive, compact, and, since a resonant antenna is employed, highly efficient.
A more complete understanding of the RFID tag employing a hollowed monopole antenna will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings which will first be described briefly.